Bulk material conveying apparatus for pneumatic dredgers are known in which a conveying line carries at its lower end a suction head or conveyor head, the conveying line comprising two telescopically slidable tube sections and a compressed air line coordinated to the conveying line and connected to either a suction head or conveyor head, thereof. Herein the upper tube section of the conveyor line is provided at its lower end and the lower tube section of the same is provided at its upper end with guide members, which coact with the corresponding other tube section and which in each case form an interspace along the overlapping parts of the tube sections. It is also known that the compressed air line comprises a rigid tube section over a length adapted to the length of the lower telescoping tube section of the conveying line, which is connected to the suction or conveyor head, respectively, and which is liftable and lowerable with the lower telescoping tube section of the conveying line.
Bulk material conveying apparatus of this kind are already state of the art, and are shown for example by the Frech Patent disclosure No. 21 26 920 and the German Auslegeschrift No. 23 55 831 (see U.S. Pat. No. 4,094,548).
It is necessary in such bulk material conveying apparatus to lift and to lower the suction head or conveyor head, respectively depending on the varying water depths encountered during the conveying process in order to maintain the bulk material suction or conveyor head at ground level along the water bed.
In order to achieve adjustability of the suction or conveyor head, the conveying lines comprises two one slidable in the other, tube sections, the relative slidability of which is such so as to balance the expected level differences at the water bed.
Difficulties have resulted in the practical operation of such bulk material conveyors in practical applications, since from the bulk material/water mixture transported, sand and gravel can pass into the interspace between the two telescoping tube sections. The result is that the relative movability of the telescoping tube sections can be blocked or at least interfered with and understandably this also results in higher wear.
In order to avoid these difficulties according to French patent disclosure No. 21 26 920, there is provided at the outer tube section an annular chamber sealed with sealing lips relative to the inner tube section which is fed with compressed air. Based on this embodiment it is possible to achieve in fact on the lower end of the outer tube section a seal against the inner tube section, since the interspace is closed off at its bottom. At the upper side of the inner tube section however no such sealing is provided with the result that from the top, bulk material can pass into the interspace when upon switching off of the conveyor pump the conveying process is interrupted. The bulk material already in the conveying line sinks against the conveying direction, downwardly, and a part of the same can pass into the open annular slot between the inner and outer tube sections.
According to German Auslegeschrift No. 23 55 831, to avoid this disadvantage, it is provided that the interspace between the two mutually overlapping telescoping tube sections forms the space to be filled with the pressure medium, and that of the sealing collars limiting this space, one of the sealing collars is at the end of one tube section and the other of the sealing collars is at the other end of the tube section.
The full length of the interspace formed by the mutually overlapping regions of the telescoping tube sections is well protected against the intrusion of bulk material in this embodiment. However, the annular sealing collars are formed from elastic material, one of which is located at the lower end of the upper outer telescoping tube section and which collar acts against the outer circumference of the lower inner telescoping tube section, and the other of which is attached to the upper end of the inner telescoping tube section and which collar acts with a sealing lip against the inner circumference of the upper outer telescoping tube section, the lifetime of the collars being very limited in view of the rough operating conditions. Especially endangered is the durability of the sharp sealing lip of the sealing collar disposed at the upper end of the inner tube section, since it is subjected to the immediate effect of the transported bulk material. Therefor, it cannot be avoided, that sand and fine grained gravel deposit between the wall of the conveyor tube and the sealing lip and generate upon lifting and lowering of the lower telescoping section, grooves and channels at the seals. These then endanger not only the sealing of the interspace filled with compressed air, but can even lead to the situation that in the course of time, bulk material passes into the interspace and slowly fills and closes and the interspace off.